1. Field of the Invention
The present invention is directed to investment casting, and more particularly, to a composition for an investment casting shell and a corresponding method that minimizes “pattern-to-pour” cycle times at a significant cost savings.
2. Description of Related Art
Investment casting is commonly used to produce high quality metal products with relatively close dimensional tolerances. The investment casting process is well known and commonly practiced. Generally, an investment casting of a part is made by creating a thin-walled ceramic mold over a previously formed pattern of the part. After the mold or shell has hardened, the pattern is melted or otherwise removed from it. The ceramic shell is then filled with a molten metal and allowed to solidify. The shell is then removed to reveal the desired part. Typically, some post-process conditioning is required to finish the part. Notably, the process may also be referred to herein with one or more of the following terms including lost wax, lost pattern, ceramic shell, or precision casting.
More particularly, as noted, investment casting methods initially include constructing a pattern of the original object to be replicated. Because this pattern will later be burned or, melted out of the shell, it must be either a low melting point substrate or a combustible substrate. Such substrates may include, for example, wax, polystyrene, plastic, or synthetic rubber.
Next, a ceramic shell is formed around the pattern. This can be accomplished by a two-step process including dipping the pattern into a slurry consisting of a mixture of liquid refractory binders, then disposing a refractory powder thereon. In particular, while the pattern is still wet, dry refractory grains are sieved onto the pattern. Such refractory materials are required to ensure that the integrity of the shell remains intact once the pattern is removed under elevated temperatures. Notably, one significant drawback of conventional investment casting is that during this stage, refractory grains, such as silica, become airborne and create a respiratory hazard to the user. As such, great care must be taken in the handling and application of refractory grains. Ventilation systems and respirators are commonly used to combat the associated dangers of airborne silica. However, such capital equipment is extremely expensive and often serves as a barrier to users who otherwise would find investment casting to be a preferred method of producing their desired part, for example, artists that want to produce a single cast of a work.
It is also notable that traditional investment casting slurries must be continuously blunged to remain viable for use as a shell material. Otherwise, the slurry “settles out” and hardens, thus becoming useless. This is a further drawback in terms of in convenience and cost (blunging machinery, labor, short shelf-life, etc.), and thus an alternate slurry composition that does not require blunging would be ideal.
An additional drawback to traditional methods is that they require the pattern to be dipped into a refractory slurry then coated with dry refractory grains, over and over, until the desired shell thickness is obtained. Typically, shells are gradually built up to a thickness of approximately ⅛ inch or more to attempt to prevent defects from appearing in the final part. Such defects typically result from the shell shrinking, sagging or cracking. It is not uncommon in industry to use seven or more layers per shell. For some applications, the coats are applied with conventional manual dipping, but for many applications, including larger volume industrial applications, such manual dipping is prohibitively impractical. As a result, costly robotic manipulators may be required for dipping, thus serving as another barrier to those users who could otherwise effectively employ investment casting methods.
Overall, the process of shell building is time consuming because each coat of slurry (each with a corresponding coat of refractory grains) must be air-dried prior to the application of subsequent coats. Notably, in this regard, known shells cannot be baked at elevated temperatures for extended periods of time without compromising the integrity of the cured shell. In the end, the process of dipping, air drying, and redipping requires twenty-four to forty-eight hours or more to complete.
After drying, the shell is heated to at least the melting or burning point of the substrate (i.e., pattern). During this step, the substrate is melted or burned away leaving only the shell and any residual substrate. The shell is then heated to a temperature high enough to flash off the residual substrate which remains in the shell.
Before the shell has cooled significantly, the molten metal will be poured into it. Various methods are used to introduce molten metal into shells including gravity, vacuum pressure, and centrifugal methods. Notably, a shell material that has a low thermal conductivity will require a lower temperature of molten metal than one that does not have a low thermal conductivity. As understood in the art, pouring molten metal at a lower than typical temperature facilitates producing parts with more detail, and yields less oxidation. Therefore, a shell material that retains more heat is typically preferred.
Upon cooling and solidification of the molten metal, the casting shell may then be removed from the casting in conventional fashion such as by hammering or sand blasting. After the shell is removed from the casting, the casting may require a cleaning or finishing step.
More specifically, in addition to physical dimensional defects, if chemicals within a shell react with the molten metal, oxides or scaling typically will be formed and must be removed. In conventional investment casting, bead blasting or other methods of abrasion are often used to remove any oxidation, flashing and/or residual shell material from the part.
Due to economic pressures, an investment casting method that is quicker than known methods, yet accurate, was desired. To decrease the cost of producing investment cast parts, the time between creating the shell and the time that the molten metal is poured into the shell, or the “pattern-to-pour” cycle time, should be minimized. Additionally, the part as cast must be free from defects in order to minimize the need for costly post-casting machining and cleanup. Furthermore, to open the technique up for use by parties with limited budgets, the need for expensive capital equipment associated with current investment casting materials and techniques should preferably be reduced or eliminated.
In addition, a shell material that does not crack or distort was desired to reduce clean-up and post-casting machining. Any such material and/or method would preferably minimize the amount of equipment needed to produce the investment cast parts in high and low volume applications, and provide a safe working environment. And, health risks should be minimized by providing a material and method that does not require the application of dry refractory grains, thus essentially eliminating airborne refractory material.